1. Field of the Invention
The present invention relates generally to the field of valves and more particularly to sealing a valve by means of packing around the stem of a valve.
2. Background Art
A major problem with valves is leakage past the valve stem and through the packing box. The packing inside the valve stuffing box is designed to compress and fit tight to prevent leaks, but it does not always work effectively. Keeping the right amount of force on the packing can be challenging. Conventional valves have a gland nut or an equivalent for compressing the packing to effect a seal. The gland nut on top of the valve must be carefully adjusted. Put too much load on the packing and it will tamp down and put too much strain on the valve stem when it needs to move. Not enough load, and leaks force their way through the stuffing box. As time goes on, the packing may consolidate anyway. Changes in temperature, constant movement of the valve stem and bolt creep each take their toll, necessitating another adjustment of the gland nut. Live-loading is a technique developed to solve this problem. It uses the forces already in play to automatically adjust the force exerted by the gland nut. Live-loading is really spring loading. Specially designed disc springs called Bellevilles are used to maintain the minimum required packing seal pressure. This prevents leakage due to aging, deterioration or thermal cycling. This provides for a degree of self-adjustment to compensate for wear. Live loading of valve packing is accomplished by storing of energy in spring (Belleville) washers. This technique puts a compressive load on the packing and maintains a relatively leak-tight seal around the stem while allowing the stem to move as necessary during opening and dosing. The number of springs varies according to the valve configuration, but it usually is possible to design a group of springs that maintains about 80% of the original design load, even when the packing has seen some consolidation while in service. Packing material should remain flexible throughout the life of the valve. After initial torque, the packingxe2x80x94especially in high temperature applicationsxe2x80x94will relax radially and axially and must be manually adjusted to effectively seal. With live loading, seal adjustment is done automatically. The use of springs to live-load a valve also provides a guide to help the user determine if there is sufficient bolt torque on the packing. The visual flat position of the spring will indicate that the springs are fully loaded. The visual position of the spring being relaxed indicates torque loss or insufficient torque loading on the packing. The springs produce a constant force against the packing that can be more than 20 times greater than could be applied manually with a gland nut. Even with the use of only one spring, the stored energy available to maintain the force on the packing is 4 times greater than the manual force normally used on the gland nut to maintain packing load. Live loaded valve stem packing, consists of a stack of disk springs placed under the valve""s gland nuts. The disks have a spring constant such that the packing axial stress remains within a narrowly defined band of values during its whole service life. As the packing consolidates in service, the springs expand to maintain a relatively constant load on the packing. Disc springs can be stacked in parallel, nested over each other, or placed in series opposing each other. Each arrangement gives a different spring characteristic, of course.
The present invention provides live loaded valve stem packing wherein grafoil packing is molded into a metal cap which not only prevents grafoil packing from extruding after it is compressed into the packing box, but also has the feature of live loading built into the metal cap. This metal cap can be made from two different materials. The first one is the conventional flat spring materials (17-7PH S.S. or 302 S.S.) used by the Belleville washer manufacturers. The second one is a Shape Memory Alloy {commonly known as SMA materials, e.g. Nixe2x80x94Ti (Nickel-Titanium alloy) or Fexe2x80x94Sixe2x80x94Mn (Iron-Silicon-Manganese alloy). The conventional or SMA material metal cap is designed with a singular hump, entirely along the top surface of the metal cap. On this hump, there are two slots, 180 degrees apart, through the hump for the purpose of removing the packing after it is installed. This hump is the live loading feature of the invention. After the packing is installed in the packing box in the bonnet, the packing gland is then installed to the bonnet with its four associated fasteners. Upon complete assembly of the packing box, the hump is elastically flattened out due to the controlled (bolt torque) axial loading through the fasteners. As the packing consolidates with the valve in service or the packing box components relax during thermal cycling, the hump deflects to maintain a relatively constant load on the packing. In essence it is a continuous in-service adjustment. SMA material metal caps can also be used with the same philosophy. The invention optionally employs SMA material metal caps which xe2x80x9csensesxe2x80x9d increase in temperature and reacts to these changes by reverting back to it as manufactured state (in this case the hump deflects) to provide a constant axial load on the packing so it can continue to provide its sealing function even during a fire.
It is therefore a principal object of the present invention to provide a valve stem packing assembly comprising a metal cap for the packing and having a spring-like hump which deflects during consolidation to maintain a relatively constant load on the packing.
It is another object of the invention to provide a valve stem packing assembly comprising a metal cap for the packing, the cap being made of a shape memory alloy to compensate for increases in temperature.
It is still another object of the invention to provide a live loaded valve stem packing assembly using a packing retainer metal cap having a spring-like hump serving the live loading function.